doc: Use external-unstable again in getting started guide

[sbernauer]

As we have merged stackabletech/listener-operator#244 there is no need for a special ListenerClass any more

Description

Please add a description here. This will become the commit message of the merge request later.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author - [ ] Changes are OpenShift compatible - [ ] CRD changes approved - [ ] CRD documentation for all fields, following the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide). - [ ] Helm chart can be installed and deployed operator works - [ ] Integration tests passed (for non trivial changes) - [ ] Changes need to be "offline" compatible 

# Reviewer - [ ] Code contains useful comments - [ ] Code contains useful logging statements - [ ] (Integration-)Test cases added - [ ] Documentation added or updated. Follows the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide). - [ ] Changelog updated - [ ] Cargo.toml only contains references to git tags (not specific commits or branches) 

# Acceptance - [ ] Feature Tracker has been updated - [ ] Proper release label has been added - [ ] [Roadmap](https://github.com/orgs/stackabletech/projects/25/views/1) has been updated 

[adwk67]

LGTM (haven't run the script but taken on trust!)